This invention relates generally to magnetic tunnel junction (MTJ) read heads. More particularly, it relates to a magnetic tunnel junction head in which the ferromagnetic free layer functions as a flux guide.
Magnetic tunnel junction (MTJ) devices are based on the phenomenon of spin-polarized electron tunneling. A typical MTJ device includes two ferromagnetic layers separated by a thin insulating tunnel barrier layer. One of the ferromagnetic layers has a magnetic moment free to rotate in the presence of applied magnetic fields. The other ferromagnetic layer has a magnetic moment fixed by interfacial exchange coupling with an anti-ferromagnetic layer. The insulating tunnel barrier layer is thin enough that quantum mechanical tunneling of electrons can occur between the ferromagnetic layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative orientations and spin polarizations of the two ferromagnetic layers.
MTJ devices have been proposed primarily as memory cells for solid state memory devices. The state of the MTJ memory cell is determined by measuring the resistance of the MTJ when a sense current is passed perpendicularly through the MTJ from one ferromagnetic layer to the other. The probability of tunneling of charge carriers across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of two ferromagnetic layers. The tunneling current is spin polarized, which means that the electrical current passing from one of the ferromagnetic layers, for example, the layer whose magnetic moment is fixed, is predominantly composed of electrons of one spin type (spin up or spin down, depending on the orientation of the magnetic moment of the ferromagnetic layer). The degree of spin polarization of the tunneling current is determined by the electronic band structure of the magnetic material at the interface of the ferromagnetic layer with the tunnel barrier layer. One ferromagnetic layer thus acts as a spin filter. The probability of tunneling of the charge carriers depends on the availability of electronic states of the same spin polarization as the spin polarization of the electrical current in the other ferromagnetic layer. Usually, when the magnetic moments of two ferromagnetic layers are parallel to each other, there are more available electronic states than when the magnetic moments of the two ferromagnetic layers are aligned antiparallel to each other. Thus, the tunneling probability of the charge carriers is highest when the magnetic moments of both layers are parallel, and is lowest when the magnetic moments are antiparallel. When the moments are arranged neither parallel nor antiparallel, the tunneling probability takes on an intermediate value. Thus, the electrical resistance of MTJ memory cells depends on the spin polarization of the electrical current and the electronic states in both of the ferromagnetic layers.
MTJ devices have attracted more attention since a large tunneling magneto-resistance (TMR) was found at room temperature. MTJ devices have since been used as magnetoresistive read/write heads for magnetic recording. FIG. 1A is a sectional view of a MTJ head 100 of the prior art. MTJ head 100 includes a MTJ layered structure 120 sandwiched by a top lead 116 adjacent to a top shield 118 and a bottom lead 104 adjacent to a bottom shield 102. The MTJ layered structure 120 includes a ferromagnetic free layer 106, a ferromagnetic pinned layer 110, an insulating tunnel barrier layer 108 located between the ferromagnetic free layer 106 and the ferromagnetic pinned layer 110, an anti-ferromagnetic layer 112 adjacent to the ferromagnetic pinned layer 110, and a capping layer 114 adjacent to the anti-ferromagnetic layer 112. In the MTJ head 100, the ferromagnetic free layer 106, the insulating tunnel barrier layer 108, and the ferromagnetic pinned layer 110 all have their front edges exposed at the sensing surface 122 of the head, i.e., the air-bearing surface (ABS) of the air bearing slider if the MTJ head 100 is used in a magnetic recording disk drive. Unfortunately, when the MTJ head 100 is lapped to form the sensing surface 122, it is possible that material from the ferromagnetic free layer 106 and the ferromagnetic pinned layer 110 smears at the sensing surface 122 and shorts out across the insulating tunnel barrier layer 108.
Magnetoresistive (MR) head technology has been developed to produce a MTJ head for a magnetic recording system that does not suffer the problem associated with having the edges of the MTJ layers exposed at the sensing surface. FIG. 1B is a sectional view of a flux guided MTJ head 101 having the ferromagnetic free layer 105 acting as a flux guide to direct magnetic flux from the magnetic recording medium to the tunnel junction. The flux guided MTJ head 101 includes a MTJ layered structure including a ferromagnetic free layer 105, a ferromagnetic pinned layer 109, an insulating tunnel barrier layer 107 located between the ferromagnetic free layer 105 and the ferromagnetic pinned layer 109, an anti-ferromagnetic layer 111 adjacent to the ferromagnetic pinned layer 109, and a capping layer 113 adjacent to the anti-ferromagnetic layer 111. The MTJ layered structure is sandwiched by a bottom lead 103 adjacent to a bottom shield 121 and a top lead 115 adjacent to a top shield 117. In the flux guided MTJ head 101, the front edge of the ferromagnetic free layer 105 is exposed at the ABS 123, while the front edges of the capping layer 113, the anti-ferromagnetic layer 111, the ferromagnetic pinned layer 109, and the insulating tunnel barrier layer 107 are recessed from the ABS 123 by an insulation 119.
The flux guided MTJ 101 is fabricated using a method illustrated in FIGS. 2A-2D. As shown in FIG. 2A, an electrical lead 202 is first deposited on a substrate (not shown), and a ferromagnetic free layer 204 is deposited on the electrical lead 202. An insulating tunnel barrier layer 206 is deposited on the ferromagnetic free layer 204, and a ferromagnetic pinned layer 208 is deposited on the insulating tunnel barrier layer 206. An anti-ferromagnetic layer 210 is deposited on the ferromagnetic pinned layer 208, and a capping layer 212 is deposited on the anti-ferromagnetic layer 210. All of the MTJ layers are deposited by typical vacuum deposition techniques, such as ion beam deposition, RF or DC magnetron sputtering deposition, evaporation deposition, or molecular beam epitaxy (MBE) deposition. A photoresist mask 214 is deposited on the MTJ layers to define an active region of the ferromagnetic pinned layer 208. The material in the unmasked regions of the capping layer 212, the anti-ferromagnetic layer 210, the ferromagnetic pinned layer 208 and the insulating tunnel barrier layer 206 are removed as shown in FIG. 2B using subtractive techniques, such as ion beam milling, chemically-assisted ion milling, sputter etching, and reactive ion etching, preferably ion beam milling. These unmasked regions are then refilled with an insulating material 218 as shown in FIG. 2C with a quantity of the insulating material 218 also deposited onto the top and sidewalls of the photoresist mask 214. This quantity of the insulating material 218 is removed, along with the photoresist mask 214, in a liftoff process, resulting in a structure as shown in FIG. 2D.
A problem with subtractive techniques is that the endpoint must terminate precisely within the insulating tunnel barrier layer 206. This is very difficult to achieve. For example, if ion beam milling is used, the thickness of the insulating tunnel barrier layer 206 is typically about 10 xc3x85, and the ion beam milling rates are typically between 3 xc3x85/sec and 4 xc3x85/sec, which allows an endpoint target about 2-3 seconds. Furthermore, if undermining occurs, a portion of the ferromagnetic pinned layer 208 still remains at the ABS, and shorting across the insulating tunnel barrier layer 206 can occur. If overmilling occurs, then the ferromagnetic free layer 204 is thinned or damaged, and its flux conducting efficiency is greatly reduced.
U.S. Pat. No. 5,898,547 issued to Fontana, Jr. et al. on Apr. 27, 1999, discloses a flux guided magnetic tunnel junction (MTJ) head and a method for making the head. Fontana""s flux guided MTJ head is fabricated by a method different from the method described in FIGS. 2A-2D, in which the MTJ layers are deposited in a reverse order. First, a ferromagnetic pinned layer is deposited on an anti-ferromagnetic layer, and a tunnel barrier layer is deposited on the ferromagnetic pinned layer. Next, a photoresist mask is deposited on the tunnel barrier layer to define the active region. The material in the unmasked regions is removed by ion beam milling, and the unmasked region is then refilled with an insulating material. Finally, a ferromagnetic free layer is deposited on the tunnel barrier layer, and a top electrical lead is deposited on the ferromagnetic free layer to finish the flux guided MTJ head. However, this method is difficult to implement in manufacturing because ferromagnetic free layer is composed of two sublayers deposited by two different stages with the ion beam milling taking place between these two stages. The ion beam milling can potentially contaminate the first deposited sublayer. Therefore, the ferromagnetic free layer""s integrity and thickness control may be compromised.
There is a need, therefore, for a flux guided MTJ head for recording system that overcomes the above difficulties.
A flux guided magnetic tunnel junction (MTJ) head according to a first embodiment of the present invention includes a flux guided MTJ layered structure sandwiched by two electrically conductive leads, each of which is adjacent to a shield. The flux guided MTJ layered structure includes a ferromagnetic pinned layer, a ferromagnetic free layer, an insulating tunnel barrier layer located between the ferromagnetic free layer and the ferromagnetic pinned layer, and an anti-ferromagnetic layer in proximity to the ferromagnetic pinned layer.
The ferromagnetic free and the insulating tunnel barrier layers have front edges substantially coplanar with a sensing surface or air bearing surface (ABS).
In a preferred embodiment, the ferromagnetic pinned layer includes an active region having a front edge recessed from the ABS, and a non-active region located between the active region and the ABS. The non-active region of the ferromagnetic pinned layer is formed by chemical processing, such as oxidation, nitridization or fluorination, of the material in this region to render it non-conducting. The non-active region of the ferromagnetic pinned layer is substantially non-magnetic. Alternatively, the ferromagnetic pinned layer can include an anti-parallel (AP) pinned structure containing first and second ferromagnetic pinned layers sandwiching a metal spacer layer such as Ru layer. The first and second ferromagnetic pinned layers will anti-parallel to each other due to the anti-ferromagnetic coupling between the two layers through the metal spacer layer.
The flux guided MTJ layered structure further includes a capping layer adjacent to the anti-ferromagnetic layer. The capping layer has a front edge recessed from the sensing surface and substantially coplanar with the front edge of the active region of the ferromagnetic pinned layer. Furthermore, the flux guided MTJ layered structure includes a top electrically conductive lead adjacent to the capping layer. The top electrically conductive lead can have a front edge recessed from the sensing surface and substantially coplanar with the front edge of the active region of the ferromagnetic pinned layer.
The anti-ferromagnetic layer has a front edge recessed from the sensing surface and substantially coplanar with the front edge of the active region of the ferromagnetic pinned layer. Alternatively, the anti-ferromagnetic layer includes an active region having a front edge recessed from the ABS and substantially coplanar with the front edge of the active region of the ferromagnetic pinned layer, and a non-active region located between the active region and the ABS.
The flux guided MTJ heads of the first embodiment are fabricated using a method described below according to a second embodiment of the present invention. A flux guided MTJ structure is first deposited on a substrate, which can be a bottom shield. The flux guided MTJ structure includes a ferromagnetic free layer, a ferromagnetic pinned layer located above the ferromagnetic free layer, an insulating tunnel barrier layer located between the ferromagnetic pinned layer and the ferromagnetic free layer, an anti-ferromagnetic layer located on top of the ferromagnetic pinned layer, and a capping layer located on top of the anti-ferromagnetic layer. All layers of the flux guided MTJ layered structure are deposited sequentially by typical vacuum deposition techniques, such as ion beam deposition, RF or DC magnetron sputtering deposition, evaporation deposition, or MBE deposition. A photoresist mask is deposited on the flux guided MTJ structure to define the active region for the ferromagnetic pinned layer. The material of the layers of the MTJ layered structure in the unmasked regions is removed using subtractive techniques, such as ion beam milling, chemically-assisted ion beam milling, sputter etching and reactive ion etching, preferably ion beam milling, with the endpoint terminating above the insulating tunnel barrier layer.
In a preferred embodiment, the endpoint terminates within the ferromagnetic pinned layer. The remaining portion of the ferromagnetic pinned layer in the unmasked regions is rendered substantially non-conducting by chemical processing, such as oxidation, nitridization, or fluorination, and preferably oxidation, of the ferromagnetic material of this remaining portion. After the chemical processing, the milled, unmasked regions at both sides of the capping layer, the anti-ferromagnetic layer, and the ferromagnetic pinned layer are refilled with an insulating material. Chemical processing of the remaining portion of the ferromagnetic pinned layer in the unmasked regions is controlled. For example, when oxidation is used, the oxidation rates are typically between 1 xc3x85/min and 5 xc3x85/min.
Chemical processing of the non-active region of the ferromagnetic pinned layer is performed at a low rate so that the photoresist mask can survive the chemical processing. The photoresist mask is necessary in the subsequent deposition of the insulating material in the milled, unmasked regions.
Alternatively, the endpoint can terminate within the anti-ferromagnetic layer. In this case, the remaining portions of the anti-ferromagnetic layer and portions of the ferromagnetic pinned layer in the unmasked regions are rendered substantially non-conducting by chemical processing of the material of these portions. After chemical processing, the milled, unmasked regions at both sides of the capping layer and the anti-ferromagnetic layer are also refilled with an insulating material.
When the ferromagnetic pinned layer includes an AP pinned structure, the endpoint preferably terminates within the ferromagnetic pinned layer located further from the anti-ferromagnetic layer. The remaining portion of this ferromagnetic pinned layer is rendered substantially non-conducting by the chemical process.
A quantity of the insulating material is also deposited on the top and the sidewalls of the photoresist mask and is removed, along with the photoresist mask, in a liftoff process.
One advantage of the method of the present invention is that the rate of the chemical process of the non-active region of the ferromagnetic pinned layer is on the order of xc3x85/min, which is controllable, instead of xc3x85/sec. Therefore, a precise method to create the non-active region of the ferromagnetic pinned layer is now realizable. Furthermore, a key feature of this method is that the insulating tunnel barrier layer acts as a barrier to the chemical processing and therefore protects the ferromagnetic free layer from the chemical processing. In addition, another advantage of the method of the present invention is that the formation of the flux guide is achieved with an in-situ deposition of the MTJ layers.